Firefox keeps updating thumbnails even after extra tweaking

[poseidon97]

Hi there!
I'm running FireFox 65.0.x on Windows 10.
I created a number of tiles in the New Tab 'top sites' section and pinned them so they wouldn't get replaced. Clearly, pinning the tiles wasn't enough to prevent Firefox from auto-updating thumbnails linked to the tiles. Under 'about:config', I set 'browser.pagethumbnails.capturing_disabled' to 'false'. The browser keeps updating thumbnails on the New Tab, however some web-sites now get locally stored logos available via 'xul.dll'. Is there a way to block auto-updating thumbnails once and for all?